Skip to main content

Full text of "Microcontroller Based Novel Dc-to-Ac Grid Connected Inverter Topology"

See other formats

ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 201 1 

Microcontroller Based Novel Dc-to-Ac Grid 
Connected Inverter Topology 

Adil Sarwar 1 , M.S. Jamil Asghar 2 and Farhad I. Bakhsh 3 

'• 2 - 3 Department of Electrical Engineering, Aligarh Muslim University, Aligarh, India 

Email 1 : adilsarwar 1 23 @ 

Abstract — The conventional line commutated ac-to-dc 
converters/ inverters have square-shaped line current which 
contains higher-order harmonics. Moreover, it requires a 
costly and bulky dc inductor or choke. The line current with 
the high harmonic contents generates EMI and therefore it 
causes more heating of the core of distribution or power 
transformers. Alternatively, PWM based inverters using 
MOSFET/IGBT switches can be used for the above purpose. 
However, apart from higher switching losses, the power 
handling capability and reliability of these devices are quite 
low in comparison to thyristors/ SCR. A thyristor based forced 
commutated inverters are not suitable for PWM applications 
due to the problems of commutation circuits. A pure sinusoidal 
voltage output or waveform with low harmonic contents is 
most desirable in the conversion from dc to ac. In the present 
work, a novel two pulse line commutated inverter is been 
proposed with control signal generated from PIC 16F 877A. It 
improves the wave shape hence it reduces the total harmonic 
distortion (THD) of the grid interactive-inverter. The 
simulation of the circuit is done using SEVIULINK. Moreover, 
the performance of the proposed circuitry is far better than 
the conventional line-commutated inverter. It reduces THD, 
number of thyristors and dispenses with the bulky dc inductor/ 
choke. A prototype model is developed for discontinuous line 
current mode. The results are also compared with the 
simulation results in SrMULINK/ MATLAB. 

Index Terms — grid connected inverter, total harmonic distortion 
(THD), simulink, line current 

I. Introduction 

Power electronic converters and controllers are 
extensively used for different types of domestic, agricultural 
and industrial applications. Ac-to-dc converters are widely 
used for the dc voltage and power control e.g. charging of 
batteries in inverters, UPS, cell-phones and speed control of 
dc motors etc. [1-2]. A conventional thyristor or power 
semiconductor device based ac-to-dc converter, with a dc 
source and an inductor or highly inductive load at the load 
side, operates in inversion mode when the switching angle 
exceeds 90°, known as grid-connected inverter [3-7]. Thus 
power flow takes place from dc source to ac grid. These ac- 
to-dc converters are also called controlled rectifiers. The 
average power flow through them is unidirectional as well as 

However, the conventional thyristor based converters or 
rectifiers introduce substantial higher-order harmonics in the 
line current. This is the main drawback of these converters. 
Due to an inductor at the load side, there is very small ripple 

©2011 ACEEE 

in the load current (dc side) and it has almost constant 
magnitude. Therefore, the line current (ac side) has square 
shape and the total harmonic distortion (THD) is very high. 
It causes electromagnetic interference (EMI) to the 
neighboring communication networks, computer networks 
or lines (LAN/ WAN) and overheating of the core of 
distribution transformers. Therefore, in general, due to 
presence of higher order harmonics in the line current, square 
wave circuit topology although simple, is not commonly 
adopted for dc-to-ac power inversion and to feed power 
tapped from various energy sources to ac grid. Moreover, in 
this case, the load current is high but its ripple is very small. 
Therefore, the magnitude of magnetic-field intensity (H) in 
inductor remains high (with small variation due to small 
current ripple). As the load current (hence H) does not go 
back to zero level, therefore, no resetting of core takes place 
and the core offers very low effective inductance. Thus a 
bulky inductor is required at the load or dc side. It increases 
the cost, size and weight of the conventional dc-to-ac inverter. 
In this paper the discontinuous phase control switching 
technique [1], [8] is extended for a novel dc-to-ac Controlled 
Inverter using a centre-tapped transformer. Instead of one 
inductor and one dc source at load side or dc side, here two 
branches are put at load side or dc side, where each branch 
consists of one inductor and one dc source. The thyristors 
are triggered or switched using a controller circuit. One 
thyristor with an inductor and a dc source forms a positive 
load branch and it is switched in positive-half cycles of ac 
grid only. Similarly, another thyristor with another inductor 
and another dc source forms a negative load branch and it is 
triggered or switched in negative-half cycles of ac grid only. 
Each load branch is connected between one terminal of the 
secondary winding and central tapping of the centre-tapped 
transformer. The dc side load current which flows in each 
load branch is a half-wave and discontinuous current. 
Therefore, wide variation of magnetic-field intensity (H) hence 
flux or flux-density (B) takes place. Therefore, the variations 
in B and H complete half of the B-H loop. At high value of H, 
with almost constant magnitude, the slope on B-H is small. 
The effective inductance also becomes small which is 
proportional to the slope on B-H curve. Therefore, for a dc 
load current, the size of inductor increases enormously which 
increases cost, size, weight and losses in the inductor. Here, 
since partial reset or reversal of flux in B-H loop takes place 
due to half-wave dc current of the inductor windings, 
therefore the effective inductance becomes high. Although 
the current through the half winding of the secondary side of 




ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 201 1 

the centre-tapped transformer is discontinuous and its shape 
is like a half- wave sinusoidal current, but the net line current 
on the primary side of the centre-tapped transformer or ac 
grid becomes continuous. Thus, THD of the line current of 
ac grid becomes very low, contrary to the conventional 
thyristor based dc-to-ac inverters, where due to constant 
magnitude load current or dc current, the line current has 
almost square-shape and THD remains high. Moreover, in 
this proposed method the effective inductance of the inductor 
increases thus cost, size, weight and losses in the inductors 

II. Components Of A New Dc-To-Ac Controlled Inverter 

The novel dc-to-ac Controlled Inverter comprises of an 
ac grid, a controller circuit and a pair of thyristors with two 
inductors, two independent dc sources and a centre-tapped 
transformer. The controller circuit gives command signals 
which are in the form of synchronized pulses for the thyristors 
and ultimately controls the real power flow to the ac grid. 

T E 

Figurel. A New ac-to-dc Controlled Inverter circuit topology 

A full-wave converter circuit with RLE load works in two 
modes of operation i.e. rectification mode and inverter mode. 
It works in inversion mode when the switching angle is greater 
than 90° [1-2], [9]. Here, a new circuit topology is proposed as 
shown in Figure 1 . Although two dc sources or a dc source 
with center-tap arrangement are needed but it dispenses with 
a bulky and costly dc inductor. In the proposed circuit, the 
line current flowing through the inductor is bi-directional (or 

When the circuit works in inverter mode, the dc source 
transfers power to the ac source. The major advantage of the 
proposed configuration is that in discontinuous mode of 
operation, the waveform resembles a sinusoidal wave with 
low harmonic contents. Moreover, the proposed 
configuration is suitable for solar PV based power generation 

III. Analysis Of A New Dc-To-Ac Controlled Inverter 

In general, the load current can be either continuous or 
discontinuous. In the case of continuous current operation, 
the current of both thyristors overlaps. It depends upon load 
voltage (PV voltage which depends on insulation and tem- 
perature), phase angle of load or inductor (<|)) and the 

©2011 ACEEE 

switching angle [1-2], [8-11]. In the case of discontinuous 
current operation, the waveform of load current depends upon 
both the switching angle and load circuit parameters. The 
output waveform of current resembles more of a sine wave. 

A) Discontinious Mode 

In the positive half cycle the thyristor; T { is triggered at 
an angle 'a' . The conduction diagram of thyristor T { is shown 
in Figure 2. During negative half cycle the thyristor; T is 
triggered at an angle 'ti + a'. The conduction diagram of 
thyristor T is shown in Figure 3. 



L R 

Figure 2. Conduction diagram of the dc-to-ac Controlled Inverter 
during positive half cycle 

Figure 3. Conduction diagram of the dc-to-ac Controlled Inverter 
during negative half cycle 

The expression of the line current is given by [1]: 

L — + iR = V cos cot + E 

For rot = 9 and m = (E/V ), it gives, 


m / -Ca-n} \ -cs-eq 

I-i = cos(6-tp) H rr*l i-e*™W l-cos(tp-a)*& tan c>pD 


For conduction of T2 in negative half cycle, the expression 
of line current is given by: 

1 2 = — cos (_8 — (p — ji) 



g tenia?} J -|- 

COS (<p — a) * B tanCp] 

The net line current, i is equal to i{H 2 




ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 201 1 

B. Continuous Mode 

During continuous conduction mode, the thyristor T2 
starts conduction before the commutation of Tl . Under this 
condition a transient condition exists for a short duration. 
The load current is the sum of currents flowing through the 
thyristors. Under steady-state condition, the current through 
load (inductor) resembles a pure sine wave. But the loop 
currents (i l and i.) become very high which may even lead to 
failure of thyristors. Moreover, inductor supplies VAR only 
and no real power is supplied to the grid. 

IV. Simulation Of The Novel Dc-To-Ac Controlled 

A. Simulation Model 

Simulink model for proposed topology is shown in Figure 
4. Resistance is included in series with the inductor to simulate 
the real inductor. The value of inductance is 0.05 H. The 
series resistance is 0.2 ohms. The centre tap transformer has 
a ratio 230 V :: 150-0-150 V Triggering pulses are given from 
pulse generator block set of simulink library. For a practical 
circuit these pulses need to be synchronized with the grid 
voltage waveform and pulses should be generated (with 
delay) at every zero crossing of the grid voltage. The dc 
battery voltage is varied and reading taken for three different 
battery voltages (24 V, 36 V, 48 V). For each battery voltage 
the switching angle is varied from 105 degrees to 165 degrees 
and THD and power transfer variation for different 
combination of switching angles as shown in Figure 5 and 
Figure 6 respectively. 

.' . : -:;- - ■ ,■ ■'• 

:; .:■>:■:;: .■;?: 



Figure 4. MATLAB Simulation model of A New dc-to-ac Con- 
trolled Inverter 

©2011 ACEEE 

B. Simulation Results 

The simulation results obtained using simulink model is 
as shown in Figure 5 and Figure 6. 




24 Volts 
36 Volts 

4S Volts 

90 140 

switching angle (degrees) 


Figure 5. THD of line current versus switching angle 

switching angle [degrees 

24 Volts 

35 Volts 

- 48 Volts 


Figure 6. Power transferred to grid versus switching angle 

? 5 

i - 

Ff T v»m*w. i sr 50 tjttn sii*!«(*d sign* 


0.01 O.K 108 HO*,. 0,05 0.CS SJ07 OIK 

5* 100 ISO HX KO 300 350 +00 450 M0 
Fiaqwrsy ■ Hji 

Figure 7. Line current with harmonic contents in Transformer less 

grid interactive inverter system. Switching angle is 120 degrees and 

battery voltage is 48 V 

Variation of THD shows an increasing trend with increasing 
switching angle whereas the power transferred to grid de- 
creases with increase of switching angle. Thus, the inverter 
should be designed to switch at switching angle close to 90 
degrees for optimum performance. Figure 7 shows the line 
current and its harmonics as obtained on power GUI blockset 
of SIMULINK library. 



ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 201 1 

V. Experimental Setup And Results 

UJT based triggering circuit as shown in Figure 8 is 
designed to generate synchronized triggering pulses. TYN 
612 (Thyristor) is used for the experimental purpose. The 
battery voltage is 12 V. The centre tapped transformer ratio is 
230 V:: 50-0-50 V. Table 1 compares the experimental result 
with that of simulated result. They are in close proximity. 

Table I: Comparison of Experimental and Simulated data 

220 12-3-1? 

Figure 8. Circuit diagram of the UJT based triggering circuit 

Tek JL D W< 


MPos: 4400m* CURSOR 

Reduced Grid Voltage 

Line Current 

CH2 500mA MSJOOms 

S-Jun-11 m49 


it 7,40Qms 
+ h 1351H: 

<al 8 ' 

Ctiui I 


Cursof 2 

Figure 9 shows the line current with harmonics for switching 
angle =133°. 

Tek Jl_ 



Freq SlOHr 
fiRMS 430.0mA 


M Pes: 44QQms 





0.0O 6 

















































3 15 11 

S 6 7 
CH2 SuOmAEiw M 10tj0m< 

8-Jun-11 10:4$ 


_ _ Harmonics 
12 If HM00O0.CSV 
CH2 7 -13.* A 

Figure 9. (a) Blue wave form is the line current (switching 

angle=133 degree) with reduced grid voltage (red waveform) (b) 

Harmonics in line current 

VI. Microcontroller Based Grid Interactive Inverter 

Grid connected inverter has been practically implemented 
using PIC 16F877A microcontroller. Figure 10 shows the 
microcontroller based implementation of above topology. 
Switching pulses for the thyristors at the desired delay angle 
have been generated from PIC 16F877A microcontroller. These 
pulses are shown in Figure 1 1 . These pulses are given to 
driver circuit shown in figure. From the driver circuit, the 
switching pulses are given to thyristors. The flowchart for 
generating synchronized pulses for triggering the thyristor 
has been shown in Figure 13. The complete experimental setup 
is shown in Figure 12. 




=n E 


:i.'c :::■;:: 

Figure 10. Microcontroller based implementation of grid interac- 
tive inverter 

©2011 ACEEE 



ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 201 1 

Tek J~L 


M Pos: O.OOOi 
Reduced Grid Voltage 

G 1 



150 degrees phase lag 

< * i i i i*s i jkitwM rf F j<p » i n> tf» »j « -fcjm*.. i *f- «m» 

Figure 11. Synchronised pusles for thyristors 

Figure 12. Experimental setup for grid interactive inverter 


The proposed scheme for a novel grid interactive inverter 
configuration for power conversion in a solar PV system is 
found to work satisfactorily. In the scheme THD of the line 
current is reduced to a large extent by careful selection of the 
switching angle and operating the inverter in discontinuous 
conduction mode. Moreover, the proposed scheme utilizes 
only two thyristors instead of four (of conventional full-bridge 
inverter) and dispenses with a bulky dc choke. It reduces the 
overall cost of the inverter. 


[I] M. S. J. Asghar, Power Electronics, Prentice-Hall, New Delhi, 

[2] G. K. Dubey, S. R. Doradla, A. Joshi, and R. M. K. Sinha, 
Thyristorised Power Controllers, New Age International, New 
Delhi, 2001. 

[3] H-L. Jou, W-J. Chiang, and J-C. Wu, "A Simplified Control 
Method for the Grid-Connected Inverter With the Function of 
Islanding Detection," IEEE Transactions on Power Electronics, 
vol.23, no.6, pp.2775-2783, Nov. 2008. 

[4] B. S. Prasad, S. Iain, V. Agarwal, "Universal Single-Stage Grid- 
Connected Inverter," IEEE Transactions on Energy Conversion, 
vol.23, no.l, pp.128-137, March 2008. 

[5] T. Abeyasekera, C. M. lohnson, D. I. Atkinson, and M. 
Armstrong, "Suppression of line voltage related distortion in current 
controlled grid connected inverters," IEEE Transactions on Power 
Electronics, vol.20, no.6, pp. 1393- 1401, Nov. 2005. 
[6] H. Qian, I. Zhang, I-S. Lai, and W. Yu, "A High-Efficiency 
Grid-Tie Battery Energy Storage System," IEEE Transactions on 
Power Electronics, vol.26, no.3, pp.886-896, March 2011. 
[7] P. Ghani, V. Asadzadeh, and H. M. Kojabadi, "Implementation 
of three-phase grid-connected inverter using TMS320LF2407A 
microprocessor," 2 nd Power Electronics, Drive Systems and 
Technologies Conference (PEDSTC) 2011, Tehran, Iran, pp. 305- 
310, 16-17 Feb., 2011. 

[8] M. S. I. Asghar, "Fine power control by discontinuous phase- 
controlled switching," IEEE Transactions on Circuits and Systems 
I: Fundamental Theory and Applications, vol.46, no.3, pp.402- 
405, Mar 1999. 

[9] G Shen, I. Zhang, X. Zhu, and D. Xu, "Alow cost solution to 
grid-connected distributed generation inverters," 2009 IEEE 6th 
International Power Electronics and Motion Control Conference 
(IPEMC V9), Wuhan, China, pp.706-711, 17-20 May, 2009. 
[10] B. Yang, L. Wuhua, Y.Zhao, andX. He, "Design and Analysis 
of a Grid-Connected Photovoltaic Power System," IEEE Trans, on 
Power Electronics, vol.25, no.4, pp.992-1000, April 2010. 

[I I] M. E. Ropp, and S. Gonzalez, "Development of a MATLAB/ 
Simulink Model of a Single-Phase Grid-Connected Photovoltaic 
System," IEEE Trans, on Energy Conversion, vol.24, no.l, pp. 195- 
202, March 2009. 

©2011 ACEEE